Three important hypotheses will be tested related to gastric bypass surgery for the treatment of severe obesity: 1) Roux-en-Y gastric bypass surgery is an effective and safe procedure to reduce weight as shown by lower long-term total and disease-specific mortality rates compared to severely obese patients who have not had gastric surgery for treatment of obesity; 2) Roux-en-Y surgery significantly lowers long-term morbidity compared to matched severely obese patients who have not had surgery; 3) Significant weight loss after Roux-en-Y surgery will be maintained over a three year follow-up period. To test the mortality hypothesis, a consecutive series of 8,139 Roux-en-Y gastric bypass surgery patients who had surgery between 1980-1997 will be used. Two control series each of 1,000 unrelated severely obese controls will be selected for mortality comparisons from: 1)a database of 120,000 Utah families collected from 1983-1997; and 2) persons desiring surgery but refused (mostly for lack of health insurance coverage). Mean length of follow-up will be 14 years (6-24 years). Also NHANES I and II mortality data for subjects with greater than or equal to 35 BMI kg/m2 will be used as a national population-based control group. To test the morbidity and weight loss hypotheses, 400 subjects undergoing bypass surgery will be prospectively followed. Two non-surgical control groups will also be prospectively followed: 400 subjects from the Utah family database and 400 surgical refusals. These three roups will have detailed clinical visits at baseline, yearly contact with ongoing medical chart abstraction and will be re-examined after 3 years of follow-up. Because of inadequate or nonexistent control groups and limited sample size of surgical patients in nearly all studies adequate estimates of risks and benefits of gastric restrictive surgery are not available. Other than the in-progress Swedish Obesity Study, this study will be the first to include a severely obese control group for morbidity and the first to include female severely obese controls for mortality comparisons to surgical patients. Long-term follow-up of an extremely large series of surgical patients with minimal lost-to-follow-up will help define risks and benefits of this surgery. With increasing use of gastric surgery for weight control for the severely obese, it is critical to assess how effective this surgery is to reduce weight loss, improve the quality of life, and reduce morbidity and mortality.